Such an assembly is known especially in connection with the cooling of heat engines of vehicles. In these known arrangements, fluid headers are situated at the two ends of the matrix (considered in the longitudinal direction of the tubes of the latter). The headers are joined together by means of longitudinal stretchers, and the motorised fan unit is assembled to the headers or the stretchers or both. Since the base plate of the motor occupies a central position on the matrix, relatively long cross members have to be provided, whereby the base plate is fastened to the headers and/or stretchers. This adds considerably to the weight of the assembly, and also makes it difficult to obtain an assembly of sufficient rigidity.